President Barack Obama is defending his handling of conflicts in Gaza and Ukraine as fighting in both places shows no signs of letting up, despite intense U.S. diplomatic efforts this week.

At a White House briefing Friday, the president hoped to focus on positive job growth numbers. Instead, he dealt with questions on the failure of U.S. diplomatic efforts to secure a cease-fire in Gaza and stop Russia's intervention in Ukraine, where fighting raged as Russian forces continued their buildup on the border.

In Gaza, a cease-fire brokered with U.S. help collapsed moments after it was announced, when two Israeli soldiers were killed and a third was captured.

Obama said stopping the fighting is going to be a challenge. “I think it's going to be very hard to put a cease-fire back together again if Israelis and the international community can't feel confident that Hamas can follow through on a cease-fire commitment.”

When a reporter asked the president if he had lost his influence in the world, Obama said U.S. diplomatic efforts will take time, and he pointed to what he said is progress in pressuring Russia to resolve the Ukraine crisis.

European nations joined Obama this week in imposing tougher new sanctions on key Russian sectors, something the U.S. administration had been threatening for months. Short of going to war, he said there are going to be constraints in terms of what the U.S. can do to stop Russian interference in Ukraine.

“What we've done is imposed sufficient costs on Russia that objectively speaking, they should -- President Putin should -- want to resolve this diplomatically, get these sanctions lifted, get their economy growing again, and have good relations with Ukraine; but, sometimes people don't act rationally,” he said.

Obama said he called Putin on Friday and reiterated his deep concerns about Russia's increasing support for separatists in Ukraine. Obama said he told the Russian leader he prefers a diplomatic solution, and the two agreed to keep channels of communication open.